The recent escalation of the cost of energy derived from fossil fuels has stimulated strong interest in the development of alternative energy sources. Significant investments in areas such as batteries, fuel cells, hydrogen production and storage, biomass, wind power, algae, and solar energy seek to develop new ways of creating and storing energy in an economically competitive fashion. The ultimate objective is to minimize society's reliance on increasingly scarce fossil fuels and to do so in a particularly environmentally friendly way that minimizes or eliminates greenhouse gas production at a cost that beats or directly competes with fossil fuels.
A number of experts have concluded that to avoid the serious consequences of global warming, it is necessary to maintain CO2 at levels of 550 ppm or less. To meet this target, based on current projections of world energy usage, the world will need 17 TW of carbon-free energy by the year 2050 and 33 TW by the year 2100. The estimated contribution of various carbon-free sources toward the year 2050 goal are summarized below:
Projected EnergySourceSupply (TW)Wind2-4Tidal2Hydro1.6Biofuels5-7Geothermal2-4Solar600Based on the expected supply of energy from the different carbon-free sources, it is apparent that solar energy is the only viable solution to solving the global climate change problem and ridding the world of greenhouse gases. With the instant invention, the time horizon required to reap substantial benefits from solar energy will be significantly shortened.
The field of solar energy is currently dominated by solar cells constructed of crystalline silicon. Crystalline silicon, however, has a number of disadvantages as a solar energy material. First, preparation of crystalline silicon is normally accomplished through a seed-assisted Czochralski method. The method entails a high temperature melting process along with controlled cooling at near-equilibrium conditions and refining to produce a boule of crystalline silicon. Although high purity crystalline silicon can be achieved and the Czochralski method is amenable to n- and p-type doping, the method is inherently slow and energy intensive.
Second, as an indirect gap material, crystalline silicon has a low absorption efficiency. Thick layers of crystalline silicon are needed to obtain enough absorption of incident sunlight to achieve reasonable solar conversion efficiencies. The thick layers add to the cost of crystalline silicon solar panels and lead to a significant increase in weight. The increased weight necessitates bulky installation mounts and precludes the use of crystalline silicon in a number of applications.
A number of alternatives to crystalline silicon are currently being pursued in an effort to achieve more versatile, more efficient and less expensive photovoltaic materials. Among the alternatives are CdTe, CdS, and CIGS (Cu—In—Ga—Se alloys). CdTe and CdS suffer from the problem that Cd is a toxic element that requires special disposal procedures. In addition, Te is a relatively scarce element, having an abundance just below that of Pt. Although CIGS shows promising solar efficiency, it is difficult to achieve uniform stoichiometry over the large scale substrates required for high-volume manufacturing.
Amorphous silicon (and hydrogenated or fluorinated forms thereof) is an attractive alternative to crystalline silicon. Amorphous silicon is a direct gap material with a high absorption efficiency. As a result, lightweight and efficient solar cells based on thin layers of amorphous silicon or related materials are possible. The instant inventor, Stanford R. Ovshinsky, is the seminal figure in modem thin film semiconductor technology. Early on, he recognized the advantages of amorphous silicon (as well as amorphous germanium, amorphous alloys of silicon and germanium as well as doped, hydrogenated and fluorinated versions thereof) as a solar cell material and was the first to recognize the advantages of nanocrystalline silicon as a photovoltaic material. Stanford R. Ovshinsky also pioneered the continuous manufacturing techniques needed to produce thin film, flexible solar panels based on amorphous, nanocrystalline, microcrystalline, polycrystalline or composite semiconductors
Representative discoveries of Stanford R. Ovshinsky in the field of amorphous semiconductors and photovoltaic materials are presented in U.S. Pat. No. 4,400,409 (describing a continuous manufacturing process for making thin film photovoltaic films and devices); U.S. Pat. No. 4,410,588 (describing an apparatus for the continuous manufacturing of thin film photovoltaic solar cells); U.S. Pat. No. 4,438,723 (describing an apparatus having multiple deposition chambers for the continuous manufacturing of multilayer photovoltaic devices); U.S. Pat. No. 4,217,374 (describing suitability of amorphous silicon and related materials as the active material in several semiconducting devices); U.S. Pat. No. 4,226,898 (demonstration of solar cells having multiple layers, including n- and p-doped); U.S. Pat. No. 5,103,284 (deposition of nanocrystalline silicon and demonstration of advantages thereof); and U.S. Pat. No. 5,324,553 (microwave deposition of thin film photovoltaic materials) as well as in articles entitled “The material basis of efficiency and stability in amorphous photovoltaics” (Solar Energy Materials and Solar Cells, vol. 32, p. 443-449 (1994); and “Amorphous and disordered materials—The basis of new industries” (Materials Research Society Symposium Proceedings, vol. 554, p. 399-412 (1999).
Current efforts in photovoltaic material manufacturing are directed at increasing the deposition rate. Higher deposition rates lower the cost of thin film solar cells and lead to a decrease in the unit cost of electricity obtained from solar energy. As the deposition rate increases, thin film photovoltaic materials become increasingly competitive with fossil fuels as a source of energy. Presently, PECVD (plasma-enhanced chemical vapor deposition) is the most cost-effective method for the commercial-scale manufacturing of amorphous silicon and related solar energy materials. Current PECVD processes provide uniform coverage of large-area substrates with device quality photovoltaic material at a deposition rate of ˜1-20 {acute over (Å)}/s.
In order to leap beyond the present deposition rates, it is necessary to overcome basic limitations associated with current PECVD techniques. One problem with PECVD-deposited photovoltaic materials is the presence of a high concentration of intrinsic defects in the as-deposited state. The intrinsic defects include structural defects (e.g. dangling bonds, strained bonds, unpassivated surface states, non-tetrahedral bonding distortions, coordinatively unsaturated silicon or germanium) that create electronic states within the bandgap of the photovoltaic material. The midgap states detract from solar conversion efficiency because they act as nonradiative recombination centers that deplete the concentration of free carriers generated by absorbed sunlight. Instead of being available for external current, the energy of many of the photoexcited free carriers is dissipated thermally. The external current delivered by a photovoltaic material is reduced accordingly.
The intrinsic defects are also believed to contribute to a degradation of solar cell performance of silicon-based photovoltaic materials through the Staebler-Wronski effect. The Staebler-Wronski effect is a photo-induced degradation of amorphous silicon and related materials (e.g. hydrogenated, fluorinated or doped forms thereof) that can cause a decrease in solar efficiency of up to 25%. Although the origin of the Staebler-Wronski effect has not been definitively established, it is believed that the creation of additional midgap defect states due to a transfer of energy from photoexcited carriers to intrinsic structural defects is a contributing factor.
One strategy for reducing the concentration of intrinsic defects in amorphous semiconductors and other photovoltaic materials prepared by conventional PECVD is to include a defect compensating agent in the plasma. Inclusion of fluorine or high excess hydrogen in the plasma, for example, leads to a marked improvement in the quality of the material and the ability to make nanocrystalline phases. The compensating agents passivate defects, saturate bonds, relieve bond strain and remove non-tetrahedral structural distortions that occur in as-deposited material. As a result, the concentration of midgap band states is reduced and higher solar conversion efficiency is achieved.
Recognizing that the use of excess H2 leads to poor gas utilization and the formation of polysilane powders, Ovshinsky has advocated the use of fluorine. In particular, Ovshinsky has shown that the inclusion of fluorine provides more regular bonding, leads to fewer defects, and enables deposition of nanocrystalline materials. (See U.S. Pat. No. 5,103,284 (formation of nanocrystalline silicon from SiH4 and SiF4); U.S. Pat. No. 4,605,941 (showing substantial reduction in defect states in amorphous silicon prepared in presence of fluorine); and U.S. Pat. No. 4,839,312 (presents several fluorine-based precursors for the deposition of amorphous and nanocrystalline silicon)).
Although defect compensating agents improve the performance of photovoltaic materials, it has been necessary to slow the deposition process to realize their benefits. Compensation or repair of intrinsic defects requires a sufficient time of contact of the compensating agent with as-deposited photovoltaic material. It is also necessary for the compensating agents to act throughout the deposition process. When an initial layer of photovoltaic material is deposited, it includes a certain concentration and distribution of intrinsic defects. Since the defect compensation process occurs preferentially at the surface, it is necessary to expose the as-deposited material to the compensating agent before an additional thickness of photovoltaic material is deposited. If the deposition continues before the defects are compensated, the defects become incorporated within the bulk of the material and are increasingly difficult to remove by subsequent exposure to a defect compensating agent. As a result, the best quality photovoltaic material is prepared at deposition rates slow enough to insure that the defect compensating agents fully interact with the as-deposited material.
S. R. Ovshinsky has further shown that the Staebler-Wronski effect is greatly diminished in nanocrystalline silicon. Through formation of nanocrystalline silicon or a nanocrystalline phase within an amorphous silicon matrix, photodegradation and light soaking effects are greatly ameliorated. Nanocrystalline silicon provides sufficient order to minimize intrinsic defects while providing enough structural flexibility to avoid formation of a rigid crystalline silicon matrix. As a result, nanocrystalline silicon provides both a high absorption efficiency and a stable photovoltaic response.
In addition to forming materials having a low concentration of defects, it is desirable to deposit materials that exhibit favorable electronic properties. In photovoltaic materials, for example, a high collection efficiency of photoexcited carriers is desired. When incident sunlight penetrates and is absorbed by a photovoltaic material, it creates photoexcited carriers in the interior thereof by promoting electrons from the valence band to the conduction band. In order to harness electrical energy from the photovoltaic material, it is necessary for the photoexcited carriers to move through the material to an electrical contact placed at the surface of the material. The ability of photoexcited carriers to reach surface contacts depends on the concentration of defects within the material and on the transport properties of the carriers. Carrier mobility is a measure of the ease with which carriers can migrate spatially within a material and carrier collection is facilitated if the mobility of electrons and holes is high. An insufficient carrier mobility increases the likelihood that a photoexcited carrier is trapped, recombined or otherwise dissipated before it is extracted from the material to contribute to an external current.
A drawback of amorphous silicon as a solar energy material is its low hole mobility. The low hole mobility reduces the solar efficiency of amorphous silicon and makes simple p-n junction devices based on amorphous silicon impractical. Instead, more complicated p-i-n structures are needed to sweep carriers to compensate for the poor hole mobility. An alternative strategy for overcoming the poor hole mobility of amorphous silicon was invented by S. R. Ovshinsky in U.S. Pat. No. 5,103,284. This patent describes the formation of a clustered semiconductor material, with dimensions on the order of 10-50 {acute over (Å)}, that has a state of order intermediate between the extended periodic lattice structure of crystalline silicon and the random, disordered structure of amorphous silicon. Ovshinsky showed that the ordered cluster material represents a new state of matter having properties that combine features of the amorphous and crystalline phases. In particular, Ovshinsky showed that the ordered cluster material possesses the high carrier mobility of crystalline silicon and the high absorption efficiency of amorphous silicon. Ovshinsky has further developed diagnostic methods for detecting ordered clusters and has elucidated the concept of intermediate range order as a signature for high quality photovoltaic materials. Although the ordered cluster material has desirable properties, current techniques for forming it suffer from low deposition rates.
A need exists in the art for a method for preparing photovoltaic materials (including amorphous, nanocrystalline, microcrystalline, and polycrystalline forms of silicon, germanium, and alloys of either) at high deposition rates without sacrificing the photoexcitation efficiency, carrier collection efficiency, and other attributes of quality. The low deposition rates needed to achieve high efficiency photovoltaic materials through conventional PECVD limits the economic competiveness of photovoltaic materials and motivates a search for new deposition processes.